Ink jet image-forming method, ink jet color image-forming method and ink jet recording apparatus

ABSTRACT

An ink jet image-forming method for forming an image by applying an ink in a fixed amount of 0.5 pl or more to 6.0 pl or less to plain paper. The ink contains a self-dispersion pigment, water, a water-soluble compound having a coefficient of hydrophilicity-hydrophobicity of 0.37 or more as defined by the equation (A) stated in the description and a water-soluble compound having a coefficient of hydrophilicity-hydrophobicity of 0.25 or less, and has a surface tension of 34 mN/m or less. The application of the ink is divided into plural times when an image having a portion where the ink is applied in a duty of 80% or more and an amount of 5.0 μl/cm 2  or less in total is formed in a basic matrix for forming the image, and the amount of the ink applied at each of the times divided is 0.7 μl/cm 2  or less.

TECHNICAL FIELD

The present invention relates to an ink jet image-forming method, an inkjet color image-forming method and an ink jet recording apparatus, inwhich images are formed on plain paper.

BACKGROUND ART

With the spread of an ink jet recording system, there is a demand forspeeding up of recording and improvement in the quality of recordedimages, such as images of characters and photographs, when recording isconducted on plain paper by ink jet recording.

For example, there is a demand for recording office documents,photographic images of digital cameras and various kinds of informationput on internet websites on plain paper at high speed and on both sidesof the plain paper. There is also a demand for achieving such clearimage quality as in recorded images obtained with a laser beam printer.There is further a demand for obtaining a high image density whencharacter images are printed and providing sharp character images thatare inhibited from being defaced even when the character images aresmall.

In recorded images such as color photographs and tables, there is also ademand for inhibiting bleeding at a color boundary portion, which iscaused upon contact of a plurality of recording inks of different colorswith each other.

In order to meet such demands, there has been proposed an ink, whichcontains organic ultrafine particles having an average particle size of0.5 μm or less and having been internally and three-dimensionallycrosslinked and provides high-density images inhibited from bleeding onplain paper (see Japanese Patent Application Laid-Open No. 2004-195706).There has also been proposed an ink that contains apermeability-imparting agent wherein the content of thepermeability-imparting agent is made larger than the content in whichthe surface tension of the ink composition, which decreases according tothe increase in the amount of the agent, ceases to decrease (seeJapanese Patent Application Laid-Open No. 2003-301129).

DISCLOSURE OF THE INVENTION

The present invention relates to an image forming method and a recordingapparatus, which are suitable for use in forming a recorded image onplain paper. Problems sought for achievement are shown below.

-   1) An ink is fixed at high speed.-   2) A recorded image has a high density and is clear.-   3) A recorded image is inhibited from causing bleeding between inks.-   4) A recorded image is inhibited from causing strike through.-   5) Even when small characters are printed, the printed characters    are sharp and inhibited from being defaced.-   6) A recorded image has good water fastness and fixability.-   7) Ejection properties such as inhibited clogging are good.

According to inks described in Japanese Patent Application Laid-OpenNos. 2004-195706 and 2003-301129, water fastness is achieved, andbleeding between two (2) color inks is prevented to some extent.However, with these inks, all the above seven problems are notsatisfied, and in particular, problems of making the density of recordedimage higher, inhibiting the strike through of the recorded image andachieving good printing of small characters are left unsolved.

As described above, an ink jet image-forming method capable ofsufficiently satisfying all the above seven problems, which have beendemands for achieving high-quality recording on plain paper in recentyears, at the same time is not found.

Accordingly, it is an object of the present invention to provide an inkjet image-forming method, an ink jet color image-forming method and anink jet recording apparatus, which sufficiently satisfy the aboveproblems 1) to 7) at the same time.

The above object can be achieved by the present invention describedbelow. More specifically, according to a first invention, the presentinvention provides an ink jet image-forming method for forming an imageby applying an ink in a fixed amount of from 0.5 pl or more to 6.0 pl orless to plain paper, wherein the ink comprises a self-dispersionpigment, water, a water-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more as defined by thefollowing equation (A) and a water-soluble compound having a coefficientof hydrophilicity-hydrophobicity of 0.25 or less as defined by thefollowing equation (A), and has a surface tension of 34 mN/m or less,and the application of the ink is divided into plural times when animage having a portion where the ink is applied in a duty of 80% or moreand in an amount of 5.0 μl/cm² or less in total is formed in a basicmatrix for forming the image, and the amount of the ink applied at eachof the times divided is 0.7 μl/cm² or less.

Coefficient of hydrophilicity-hydrophobicity=[(Water activity value of a20% aqueous solution)−(Molar fraction of water in the 20% aqueoussolution)]/[1-(Molar fraction of water in the 20% aqueoussolution)]  Equation (A)

According to a second invention, the present invention provides an inkjet color image-forming method for forming an image by applying each ofat least 4 color inks of black, cyan, magenta and yellow in a fixedamount of from 0.5 pl or more to 6.0 pl or less for each ink to plainpaper, wherein each of the 4 color inks comprises a self-dispersionpigment, water, a water-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more as defined by the equation(A) and a water-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.25 or less as defined by the equation(A), and has a surface tension of 34 mN/m or less, and the applicationof the inks is divided into plural times when an image having a portionwhere the inks are applied in a duty of 80% or more and in an amount of5.0 μl/cm² or less in total is formed in a basic matrix for forming theimage, and the amount of the inks applied at each of the times dividedis 0.7 μl/cm^(2 or less.)

According to a third invention, the present invention provides an inkjet recording apparatus equipped with a recording head for forming animage by applying an ink in a fixed amount of from 0.5 pl or more to 6.0pl or less to plain paper, wherein the apparatus comprises a controlmechanism, by which the application of the ink is divided into pluraltimes when an image having a portion where the ink is applied in a dutyof 80% or more and in an amount of 5.0 μl/cm² or less in total is formedin a basic matrix for forming the image, and the amount of the inkapplied at each of the times divided is controlled to 0.7 μl/cm² orless, and the ink comprises a self-dispersion pigment, water, awater-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more as defined by the equation(A) and a water-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.25 or less as defined by the equation(A), and has a surface tension of 34 mN/m or less.

EFFECTS OF THE INVENTION

According to the ink jet image-forming method, ink jet colorimage-forming method and ink jet recording apparatus of the presentinvention, the clogging with an ink can be inhibited, and the fixing ofthe ink can be conducted at high speed when the ink is applied to plainpaper. It is also possible to provide a clear and high-quality imagehaving sufficient water fastness and image density and inhibited frombleeding. Even when small characters are printed, the resultingcharacters are sharp and can be inhibited from being defaced. It isfurther possible to form an image inhibited from causing strike throughand suitable for double sided printing. These are marked effects thatare exhibited for the first time by meeting all the above-describedconstitutional requirements of the present invention and unanticipatablefrom the prior art.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an ink jet recording apparatusapplicable to the present invention.

FIG. 2 illustrates recording heads applicable to the present invention.

FIG. 3 illustrates an example of an image forming method according tothe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors have carried out an investigation on ink jetrecording method and apparatus that provide clear and high-qualityimages fixed at high speed to plain paper and having sufficient waterfastness and image density, prevent clogging upon ejection and aresuitable for double side printing. As a result, it has been found thatthe composition of an ink that quickly causes solid-liquid separationbetween a pigment and an aqueous medium after impacted on the plainpaper, physical properties of the ink, the amount of the ink to beapplied, which is controlled on the side of a recording apparatus, andconditions for divided application of the ink are precisely controlled,whereby the above object can be achieved by the synergistic effect ofthese.

The present invention will hereinafter be described in more detail bypreferred embodiments.

In the present invention, the amount of an ink applied at one time iscontrolled to a fixed amount of from 0.5 pl or more to 6.0 pl or less.The amount is preferably 1.0 pl or more, more preferably 1.5 pl or more.Further, the amount is preferably 5.0 pl or less, more preferably 4.5 plor less. Any amount less than 0.5 pl is not preferable because theresulting image may become poor in fixability and water fastness in somecases. If the amount exceeds 6.0 pl, characters printed may be defacedby dot gain in some cases when small characters of the order of from 2point (1 point ≈0.35 mm) to 5 point are printed.

Since the volume of the ink ejected greatly affects the strike throughof the ink, the volume is important even in respect of the applicationto double side printing. Generally, pores having a size of from 0.1 μmto 100 μm centering around from 0.5 μm to 5.0 μm are distributed inplain paper. Incidentally, the plain paper referred to in connectionwith the present invention means copying paper used in a large amount inprinters and copying machines, such as commercially available wood freepaper, medium grade paper and PPC paper, or bond paper. The permeationphenomenon of an aqueous ink into the plain paper is largely dividedinto fiber absorption that the ink is directly absorbed and permeatedinto the cellulose fiber itself of the plain paper and pore absorptionthat the ink is absorbed and permeated into pores formed betweencellulose fibers. The inks used in the present invention are inks to bemainly permeated by the pore absorption, which will be described later.Therefore, when the ink used in the present invention is applied to theplain paper and a part of the ink comes into contact with largish poresof about 10 μm or more, which are present on the surface of the plainpaper, the ink is concentrated on the largish pores according to theLucas-Washburn equation and absorbed and permeated therein. As a result,in this portion, the ink comes to particularly deeply be permeated,which is extremely disadvantageous in achieving high color developmenton the plain paper. On the other hand, as an ink becomes smaller in thesize of ink droplet, the probability of an ink droplet coming intocontact with the largish pore becomes lower, so that the ink is hard tobe concentrated on and absorbed in the largish pore. Further, even ifthe ink droplet comes into contact with the largish pore, the amount ofthe ink deeply permeated may be small so far as the ink droplet issmall. As a result, an image with high color development is obtained onthe plain paper.

The upper critical value of 6.0 pl of the ink is a value empiricallyobtained by the present inventors. When 6.0 pl of the ink is assumed tobe a sphere, the diameter thereof is about 23 μm when impacted on theplain paper. Taking the distribution condition of the largish pores ofabout 10 μm or more in the plain paper into consideration, it isconsidered that when the diameter of the ink is this diameter or less,the probability of contact between the largish pores and the ink uponimpact becomes low, and so a preferable state without causing deeppermeation of the ink is created.

The fixed amount of the ink, referred to in connection with the presentinvention means an ink ejected in a state that the structures of nozzlesmaking up a recording head are not made different from among the nozzlesand the setting of changing drive energy applied is not made. In otherwords, in such a state, the amount of the ink applied is fixed even ifejection is somewhat varied because of an error in production ofapparatus. With the amount of the ink applied made fixed, the permeationdepth of the ink is stabilized, the image density of a recorded imagebecomes high, and image uniformity is improved. On the contrary, theamount of the ink is not fixed according to a system based on thepremise that the amount of the ink applied is varied, so that variationin permeation depth of the ink becomes great because inks different involume mixedly exist. In a high duty portion of a recorded image inparticular, the image uniformity is deteriorated because a portion lowin image density exists in the recorded image due to the variation inthe permeation depth.

As a system suitable for applying an ink in a fixed amount, a thermalink jet system that the ink is applied by the action of thermal energyis preferred from the viewpoint of ejection mechanism. Morespecifically, according to the thermal ink jet system, the variation inpermeation depth of the ink is inhibited, and the resulting recordedimage is high in image density and good in uniformity. In addition, thethermal ink jet system is suitable for achieving a recording head of amulti-nozzle and high-density type and also preferable for high-speedrecording.

The problems of the present invention are then required in the casewhere an image having a portion with a duty of 80% or more is formed ina basic matrix for forming an image. A portion for calculating the dutyis 50 μm×50 μm at the minimum. The image having a portion with a duty of80% or more is an image having a portion formed with an ink applied to80% or more of lattices among lattices in the matrix of the portion forcalculating the duty. The size of lattices is determined by theresolution of the basic matrix. For example, when the resolution of thebasic matrix is 1,200 dpi×1,200 dpi, the size of a lattice is 1/1,200inch× 1/1,200 inch.

The image having a portion with a duty of 80% or more in the basicmatrix is an image having a portion where one color ink is applied in aduty of 80% or more in the basic matrix. More specifically, in the casewhere four (4) color inks of black, cyan, magenta and yellow are used,the image means an image having a portion where at least one color inkthereof are applied in a duty of 80% or more in the basic matrix. On theother hand, an image having no portion with a duty of 80% or more in thebasic matrix has a relatively small overlapping between inks impactedand may not cause problems of character defacement and bleeding in manycases even when the printing process is not devised.

The basic matrix of the present invention can be freely set according tothe recording apparatus. The resolution of the basic matrix ispreferably 600 dpi or more, more preferably 1,200 dpi or more. Also, theresolution is preferably 4,800 dpi or less. The resolution may be thesame or different in length and width so far as it falls within thisrange.

The problems of the present invention are also required in the casewhere an image having a portion wherein the total amount of an inkapplied is 5.0 μl/cm² or less is formed in a basic matrix for forming animage. A portion for calculating the total amount of the ink applied isthe same as the portion for calculating the duty. If an image having aportion that the total amount of the ink applied exceeds 5.0 μl/cm² isformed, in some cases, a clear image may not be obtained, or strikethrough may occur, which is disadvantageous for double side printing.

In the present invention, the application of the ink is divided intoplural times when an image having a portion where the duty is 80% ormore and the total amount of an ink applied is 5.0 μl/cm² or less isformed in the basic matrix for forming the image. The amount of the inkapplied at each of the times divided is 0.7 μl/cm² or less, preferably0.6 μl/cm² or less, more preferably 0.5 μl/cm² or less. If the amount ofthe ink applied in the image at each time exceeds 0.7 μl/cm², strikethrough, character defacement and/or bleeding may occur in some cases.

The reason why the application of the ink divided into the plural timesin the formation of the image is an essential requirement in the presentinvention is that there is a particular difference in performancebetween the case where the application is divided and the case where theapplication is not divided. The number of times of division of the inkapplication is at least 2 times or more. When the number of times ofdivision is 3 times or more, the resulting recorded image becomes higherin density and good color developing. The number of times of division ispreferably 8 times or less, more preferably 4 times or less. If thenumber of times of division exceeds 8 times, there is a tendency thatthe covering rate of the ink on the surface of plain paper is lowered todeteriorate color development.

The inks and ink jet recording apparatus according to the presentinvention will now be described.

<Ink> (Coloring Material)

Coloring materials used in inks in the present invention areself-dispersion pigments. An ink set used in the formation of a colorimage with inks of a plurality of colors is basically composed of black,cyan, magenta and yellow inks. However, red, blue, green, gray, lightcyan and light magenta inks may be added as needed. Pigments containedin these inks are also preferably self-dispersion pigments. When aself-dispersion pigment is used in an image forming process related tothe present invention, good water fastness is exhibited. Theself-dispersion pigment develops a synergistic effect with thewater-soluble compounds used in combination in the present invention tosmoothly advance solid-liquid separation after the ink is impacted onpaper, thereby making color developing excellent. When theself-dispersion pigment is used in the present invention, the pigmentacts synergistically with the conditions for application of the ink,whereby smooth solid-liquid separation proceeds as compared with, forexample, the case where a resin-dispersion pigment is used, and thepigment itself is hard to deeply permeate into the interior of plainpaper, so that the color developing performance is very improved.

The self-dispersion pigment is a pigment which does not basicallyrequire a dispersant as an essential and is made water-soluble byintroducing a water-soluble functional group into the surface of thepigment directly or through another atomic group. As pigments beforemade water-soluble may be used various pigments.

As a pigment used in a black ink, carbon black is preferably used.Examples of carbon black include carbon black pigments such as furnaceblack, lamp black, acetylene black and channel black. Such a carbonblack pigment preferably has the following characteristics: the primaryparticle size is from 15 nm or more to 40 nm or less; the specificsurface area is from 50 m²/g or more to 400 m²/g or less as determinedaccording to the BET method; the DBP oil absorption is from 40 ml/100 gor more to 200 ml/100 g or less; and the volatile matter content is 0.5%by weight or more to 10% by weight of less.

As pigments used in color inks, organic pigments are preferably used. Asspecific examples thereof, may be mentioned the following pigments:insoluble azo pigments such as Toluidine Red, Toluidine Maroon, HansaYellow, Benzidine Yellow and Pyrazolone Red; water-soluble azo pigmentssuch as Lithol Red, Helio Bordeaux, Pigment Scarlet and Permanent Red2B; derivatives from vat dyes, such as alizarin, indanthron andThioindigo Maroon; phthalocyanine pigments such as Phthalocyanine Blueand Phthalocyanine Green; quinacridone pigments such as Quinacridone Redand Quinacridone Magenta; perylene pigments such as Perylene Red andPerylene Scarlet; isoindolinone pigments such as Isoindolinone Yellowand Isoindolinone Orange; imidazolone pigments such as BenzimidazoloneYellow, Benzimidazolone Orange and Benzimidazolone Red; pyranthronepigments such as Pyranthrone Red and Pyranthrone Orange; thioindigopigments; condensed azo pigments; diketopyrrolopyrrole pigments; andother pigments such as Flavanthrone Yellow, Acylamide Yellow,Quinophthalone Yellow, Nickel Azo Yellow, Copper Azomethine Yellow,Perinone Orange, Anthrone Orange, Dianthraquinonyl Red and DioxazineViolet.

When organic pigments are numerated by COLOR INDEX (C.I.) numbers, thefollowing pigment may be exemplified. C.I. Pigment Yellow: 12, 13, 14,17, 20, 24, 55, 74, 83, 86, 93, 97, 98, 109, 110, 117, 120, 125, 128,137, 138, 147, 148, 150, 151, 153, 154, 155, 166, 168, 180 and 185; C.I.Pigment Orange: 16, 36, 43, 51, 55, 59, 61 and 71; C.I. Pigment Red: 9,48, 49, 52, 53, 57, 97, 122, 123, 149, 168, 175, 176, 177, 180, 192,202, 209, 215, 216, 217, 220, 223, 224, 226, 227, 228, 238, 240, 254 and272; C.I. Pigment Violet: 19, 23, 29, 30, 37, 40 and 50; C.I. PigmentBlue: 15, 15:1, 15:3, 15:4, 15:6, 22, 60 and 64; C.I. Pigment Green: 7and 36; and C.I. Pigment Brown 23, 25 and 26. Among these pigments, C.I.Pigment Yellow: 13, 17, 55, 74, 93, 97, 98, 110, 128, 139, 147, 150,151, 154, 155, 180 and 185 as yellow pigments, C.I. Pigment Red: 122,202 and 209, and C.I. Pigment Violet 19 as magenta pigments, and C.I.Pigment Blue: 15:3 and 15:4 as cyan pigments are more preferable. Quitenaturally, other pigments than the above-mentioned pigments may also beused.

The hydrophilic group introduced into the self-dispersion pigments usingthe above pigments as a raw material may be bonded directly to thesurface of the pigments. Alternatively, the hydrophilic group may bebonded indirectly to the surface of the pigment by interposing thehydrophilic group between another atomic group and the surface of thepigments. Examples of an anionic functional group introduced and bondedinclude the following groups: hydrophilic groups such as —COO(M),—SO₃(M) and —PO₃(M)₂ (wherein M in the formulae is a hydrogen atom,alkali metal, ammonium or organic ammonium). Specific examples of thealkali metal represented by “M” in the hydrophilic groups include Li,Na, K, Rb and Cs. Specific examples of the organic ammonium include thefollowing ammonium groups: methyl- ammonium, dimethylammonium,trimethylammonium, ethyl- ammonium, diethylammonium, triethylammonium,monohydroxy-methyl(ethyl)amine, dihydroxymethyl(ethyl)amine andtrihydroxymethyl(ethyl)amine.

Specific examples of another atomic group interposed include linear orbranched alkylene groups having 1 to 12 carbon atoms, a substituted orunsubstituted phenylene group and a substituted or unsubstitutednaphthylene group. Examples of the substituents on the phenylene groupand naphthylene group include linear or branched alkyl groups having 1to 6 carbon atoms. Specific examples of combinations of another atomicgroup and the hydrophilic group include —C₂H₄—COO(M), —Ph—SO₃ (M) and—Ph—COO(M) (wherein Ph is a phenylene group).

Specific examples of production processes for directly introducing thewater-soluble group into the surface of the pigment include a process,in which for example, carbon black is subjected to an oxidizingtreatment with sodium hypochlorite. According to this process, a —COO(M)group or lactone group, which is a hydrophilic group, can be introducedinto the surface of carbon black. In the present invention, such apigment can be particularly preferably used.

When a cationic group is introduced as the water-soluble group, such acationic group is preferably composed of, for example, at least onearomatic group of phenyl, benzyl, phenacyl and naphthyl groups, or aheterocyclic group such as a pyridyl group and at least one cationicgroup. It is more preferable that a cationic group bonded to the surfaceof carbon black is a quaternary ammonium group.

The average particle size of the self-dispersion pigment used in thepresent invention is determined by a dynamic light scattering method andis preferably 60 nm or more, more preferably 70 nm or more, still morepreferably 75 nm or more. Further, the average particle size ispreferably 145 nm or less, more preferably 140 nm or less, still morepreferably 130 nm or less. As a specific method for measuring theaverage particle size, the average particle size can be measured bymeans of FPAR-1000 (manufactured by Otsuka Electronics Co., Ltd.;analysis by a cumulant method) or Nanotrac UPA 150EX (manufactured byNIKKISO; as a 50% cumulative value) utilizing scattering of laser beam.

Two or more pigments may be used in combination in the same ink asneeded.

The amount of the above-described self-dispersion pigment added into anink is preferably 0.5% by mass or more, more preferably 1% by mass ormore, still more preferably 2% by mass or more based on the total massof the ink. Further, the amount is preferably 15% by mass or less, morepreferably 10% by mass or less, still more preferably 8% by mass orless.

(Aqueous Medium)

The ink according to the present invention contains water as anessential component, and the content of water in the ink is preferably30% by mass or more based on the total mass of the ink. Further, thecontent is preferably 95% by mass or less. In addition to water, twokinds of water-soluble compounds that are essential components are usedin combination to provide an aqueous medium. The water-soluble compoundsare miscible with water without undergoing phase separation in the formof a 20% by mass liquid mixed with water and are high in hydrophilicity.Any water-soluble compound easy to evaporate is not preferable from theviewpoints of solid-liquid separation and the prevention of clogging,and a water-soluble compound having a vapor pressure of 0.04 mmHg orless at 20° C. is preferable.

The ink according to the present invention contains, as essentialcomponents, a water-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more as defined by thefollowing equation (A) and a water-soluble compound having a coefficientof hydrophilicity-hydrophobicity of 0.25 or less as defined by thefollowing equation (A).

Coefficient of hydrophilicity-hydrophobicity=[(Water activity value of a20% aqueous solution)−(Molar fraction of water in the 20% aqueoussolution)]/[1-(Molar fraction of water in the 20% aqueoussolution)]  Equation (A)

The water activity value in the equation is represented by theexpression:

Water activity value=(Water vapor pressure of the aqueoussolution)/(Water vapor pressure of pure water).

Various methods are available as methods for measuring the wateractivity value. Although the method is not limited to any method, achilled mirror dew point measuring method among others is suitable foruse in measurement of materials used in the present invention. Valuesgiven in the present description are obtained by subjecting a 20%aqueous solution of each water-soluble compound to the measurement at25° C. by means of Aqua Lab CX-3TE (manufactured by DECAGON Co.)according to this measuring method.

According to the Raoult's Law, a rate of vapor pressure depression of adilute solution is equal to a molar fraction of a solute and has noconnection with the kinds of the solvent and the solute, so that themolar fraction of water in an aqueous solution is equal to the wateractivity value. However, when water activity values of aqueous solutionsof various water-soluble compounds are measured, the water activityvalues do not often consist with the molar fraction of water.

When the water activity value of an aqueous solution is lower than themolar fraction of water, the water vapor pressure of the aqueoussolution comes to be smaller than a theoretical calculated value, andevaporation of water is inhibited by the presence of a solute. From thisfact, it is found that the solute is a substance great in hydrationforce. When the water activity value of an aqueous solution is higherthan the molar fraction of water to the contrary, the solute isconsidered to be a substance small in hydration force.

The present inventors have paid attention to the fact that the degree ofhydrophilicity or hydrophobicity of a water-soluble compound containedin an ink greatly affects the propelling of solid-liquid separationbetween a self-dispersion pigment and an aqueous medium and various inkperformances. From this fact, the coefficient ofhydrophilicity-hydrophobicity represented by the equation (A) has beendefined. The water activity value is measured on aqueous solutions ofvarious water-soluble compounds at a fixed concentration of 20% by mass.The degree of hydrophilicity or hydrophobicity between various solutescan be relatively compared by being converted to the equation (A) evenwhen the molecular weights of the solutes and the molar fractions ofwater are different. Since the water activity value of an aqueoussolution does not exceed one (1), the maximum value of the coefficientof hydrophilicity-hydrophobicity is one (1).

The coefficients of hydrophilicity-hydrophobicity of water-solublecompounds used in ink-jet inks, which are obtained according to theequation (A), are shown in Table 1. However, the water-soluble compoundsaccording to the present invention are not limited only to thesecompounds.

TABLE 1 Coefficient of hydrophilicity- Substance name hydrophobicity1,2-Hexanediol 0.97 1,2-Pentanediol 0.93 3-Methyl-1,3- 0.90 butanediol1,2-Butanediol 0.90 2,4-Pentanediol 0.88 1,6-Hexanediol 0.761,7-Heptanediol 0.73 3-Methyl-1,5- 0.54 pentanediol 1,5-Pentanediol 0.41Trimethylolpropane 0.31 Ethyleneurea 0.30 1,2,6-Hexanetriol 0.281,2,3-Butanetriol 0.22 Sorbitol 0.21 Urea 0.20 Diethylene glycol 0.151,2,4-Butanetriol 0.15 Glycerol 0.11 Diglycerol 0.08 Triethylene glycol0.07 Polyethylene glycol −0.09 200 Polyethylene glycol −0.43 600

Water-soluble compounds having the intended coefficient ofhydrophilicity-hydrophobicity can be selected for use from among variouskinds of water-soluble compounds having suitability for ink jetrecording inks.

The present inventors have carried out an investigation as to therelationship between water-soluble compounds different in coefficient ofhydrophilicity-hydrophobicity and the performance of various ink whenthe water-soluble compounds are contained in the inks. As a result, thefollowing findings have been obtained.

Printing characteristics of small characters, such as bleeding betweentwo (2) colors and dot gain were extremely improved when a water-solublecompound high in hydrophobic tendency, which has a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more, is used. Among others,compounds having a glycol structure of a hydrocarbon having 4 to 7carbon atoms were particularly preferable. It is considered that thesewater-soluble compounds are relatively small in affinity for water,self-dispersion pigment and cellulose fiber after an ink is impacted onpaper, and so the compounds have the role of strongly propellingsolid-liquid separation between the self-dispersion pigment and theaqueous medium. Among these compounds, 1,2-hexanediol and 1,6-hexanediolare particularly preferable.

On the other hand, it is important from the viewpoint of prevention ofclogging in an orifice that solidification of an ink due to evaporationof water is hard to occur. The present inventors have found that theclogging with the ink in the orifice is prevented by using awater-soluble compound having hydration force of a certain extent ormore in combination. When a water-soluble compound having a coefficientof hydrophilicity-hydrophobicity of 0.25 or less is used in combinationwith the water-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more, these compounds cansynergistically act to inhibit bleeding and dot gain in the resultingimage and further prevent clogging with the ink in the orifice. Amongothers, glycerol, diglycerol and polyethylene glycol having a numberaverage molecular weight of 200 or more are preferable.

The total content of the water-soluble compound (A) having a coefficientof hydrophilicity-hydrophobicity of 0.37 or more and the water-solublecompound (B) having a coefficient of hydrophilicity-hydrophobicity of0.25 or less in the ink is preferably 5% by mass or more, morepreferably 6% by mass or more, still more preferably 7% by mass or more.Also, the total content is preferably 40% by mass or less, morepreferably 35% by mass or less, still more preferably 30% by mass orless. A proportion “(A)/(B)” of the content of the water-solublecompound (A) to the content of the water-soluble compound (B) in the inkis preferably 1/20 or more, more preferably 1/10 or more, still morepreferably ⅕ or more. Also, the proportion is preferably 20/1 or less,more preferably 10/1 or less, still more preferably 5/1 or less.

The ink used in the present invention preferably contains awater-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.26 or more, but less than 0.37 inaddition to these water-soluble compounds. By such composition, thewater-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.26 or more, but less than 0.37, thewater-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more and the water-solublecompound having a coefficient of hydrophilicity-hydrophobicity of 0.25or less synergistically act to well inhibit bleeding and dot gain. Inaddition, the clogging with the ink can be better prevented. The contentof the water-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.26 or more, but less than 0.37 in theink is preferably 1% by mass or more, more preferably 2% by mass ormore, still more preferably 3% by mass or more. Furthermore, the contentis preferably 30% by mass or less, more preferably 25% by mass or less,still more preferably 20% by mass or less. As the water-soluble compoundhaving a coefficient of hydrophilicity-hydrophobicity of 0.26 or more,but less than 0.37, trimethylolpropane is particularly preferable.

(Surfactant)

The ink used in the present invention preferably contains a surfactantfor achieving ejection stability with good balance. In particular, theink preferably contains a nonionic surfactant. Among nonionicsurfactants, polyoxyethylene alkyl ethers and ethylene oxide adducts ofacetylene glycol are particularly preferable. The HLB(hydrophile-lipophile balance) values of these nonionic surfactants are10 or more. The content of the surfactant used in the ink in combinationis preferably 0.1% by mass or more, more preferably 0.2% by mass ormore, still more preferably 0.3% by mass or more. Also, the content ispreferably 5% by mass or less, more preferably 4% by mass or less, stillmore preferably 3% by mass or less.

(Other Additives)

Besides the above components, for example, a viscosity modifier, anantifoaming agent, a preservative, a mildew-proofing agent, anantioxidant and a penetrant may be added as additives to the inkaccording to the present invention, as needed, to provide the ink as anink having desired physical property values.

(Surface Tension)

The surface tension of the ink used in the present invention is 34 mN/mor less. The surface tension of the ink is preferably 32 mN/m or less,more preferably 30 mN/m or less.

Since glossy paper and mat paper that are exclusive paper for ink jethave a porous ink-receptive layer formed on the surface of paper unlikeplain paper, such paper is scarcely affected by the surface tension ofan ink to rapidly advance permeation of the ink.

However, a sizing agent having a water-repellent effect is internallyand/or externally added to plain paper, so that the permeation of an inkis often inhibited. In other words, the plain paper is lower in criticalsurface tension, which is an index to whether the surface can be rapidlywetted with the ink or not, than the exclusive paper for ink jet.

When the surface tension of the ink is higher than 34 mN/m, the surfacetension comes to be higher than the critical surface tension of theplain paper, so that the plain paper is not immediately wetted even whenthe ink impacts the paper, and permeation of the ink is not rapidlystarted. When the surface tension of the ink is high, such an ink ishard to be fixed at high speed even when wettability with paper issomewhat improved to lower a contact angle between the ink and thepaper. Further, such an ink has a tendency to deteriorate the fixingability thereof. When the surface tension of the ink is 34 mN/m or less,pore absorption is mainly caused. When the surface tension of the ink ishigher than 34 mN/m, fiber absorption is mainly caused. With respect tothe absorption speed of an ink into paper by absorption of these twotypes, the pore absorption is overwhelmingly faster. Thus, an ink thatmainly causes pore absorption is provided in the present invention,thereby realizing high-speed fixing.

The ink that mainly causes pore absorption is also advantageous in thatbleeding is inhibited when recording is conducted by causing two inks ofdifferent colors to adjoin because the two inks are inhibited fromremaining at the same time on the surface of paper. This ink is alsoadvantageous in that a high image density is achieved.

On the other hand, the surface tension of the ink used in the presentinvention is preferably 20 mN/m or more from another viewpoint ofoperability of the ink. When the surface tension is 20 mN/m or more, ameniscus can be retained in an orifice, so that “ink falling” that theink comes out of an ejection opening and escapes from an orifice can beinhibited.

<Ink Jet Recording Apparatus>

The ink jet recording apparatus according to the present invention willhereinafter be described. An apparatus suitable for use in the presentinvention is an apparatus equipped with a recording head for applying anink in a fixed amount of from 0.5 pl or more to 6 pl or less. Therecording head of the ink jet recording apparatus according to thepresent invention is preferably a recording head that thermal energy iscaused to act on an ink to apply the ink. Such a recording head issuitable for forming nozzles at a high density compared with a recordinghead that an ink is ejected by using a piezoelectric element. Inaddition, the recording head is excellent in applying the ink in thefixed amount and thus excellent in that variation in permeation depth ofthe ink is reduced, the uniformity of the resulting recorded image ismade good, and an image density is made high.

With respect to the typical construction and principle of the recordinghead that thermal energy is caused to act on an ink to apply the ink,those using the basic principle disclosed in, for example, U.S. Pat. No.4,723,129 and U.S. Pat. No. 4,740,796 are preferable. This system may beapplied to any of the so-called On-Demand type and continuous type. Inparticular, the On-Demand type is more advantageous because at least onedriving signal, which corresponds to recording information and gives arapid temperature rise exceeding nuclear boiling, is applied to anelectrothermal converter arranged corresponding to a sheet or a liquidpath, in which an ink is retained, thereby causing the electrothermalconverter to generate thermal energy to cause film boiling on theheat-acting surface of a recording head, so that a bubble can be formedin the ink in response to the driving signal in relation of one to one.The ink is ejected through an ejection opening by the growth-contractionof this bubble to form at least one droplet. When the driving signal isapplied in the form of a pulse, the growth-contraction of the bubbleproperly occurs in a moment, so that the amount of the ink ejected isfixed, and the ejection of the ink, which is also excellent inresponsiveness, can be achieved. It is therefore preferable to use suchpulsed signals.

FIG. 1 is a front elevation schematically illustrating an ink jetrecording apparatus according to an embodiment of the present invention.A plurality of recording heads 211 to 214 of an ink jet system ismounted on a carriage 20. A plurality of ink ejection openings forejecting an ink is arranged in each of the recording heads 211 to 214.The recording heads 211, 212, 213 and 214 are recording heads forejecting cyan (C), magenta (M), yellow (Y) and black (K) inks,respectively.

Ink cartridges 221 to 224 are respectively constructed by the recordingheads 211 to 214 and ink tanks for feeding inks to these recordingheads.

A concentration sensor 40 is provided. the concentration sensor 40 is areflection type concentration sensor and is so constructed that thedensity of a test pattern recorded on recording medium can be detectedin a state provided on a side surface of the carriage 20.

Control signals to the recording heads 211 to 214 are transferredthrough a flexible cable 23.

A recording medium 24, to the surface of which cellulose fiber isexposed, such as plain paper is held by discharge rollers 25 viaconveyance rollers (not illustrated) and conveyed in a direction(secondary scanning direction) of an arrow by driving a conveyance motor26.

The carriage 20 is guided and supported by a guide shaft 27 and a linearencoder 28. The carriage 20 is reciprocatingly moved in a main scanningdirection along the guide shaft 27 through a drive belt 29 by driving acarriage motor 30.

A heating element (electricity-thermal energy converter) for generatingthermal energy for ink ejection is provided in the interior (liquidpath) of each of the ink ejection openings of the recording heads 211 to214. The heating element is driven based on a recording signal accordingto reading timing of the linear encoder 28 to eject and apply inkdroplets to the recording medium, thereby forming an image.

A recovery unit 32 having cap parts 311 to 314 is provided at a homeposition of the carriage 20 arranged outside a recording region. Whenrecording is not conducted, the carriage 20 is moved to the homeposition, and the ink ejection opening faces of the recording heads 211to 214 are closed by their corresponding caps 311 to 314, wherebysticking of the inks caused by evaporation of ink solvents or cloggingby adhesion of foreign matter such as dust can be prevented. The cappingfunction of the cap parts is utilized for solving ejection failure orclogging of ejection openings low in recording frequency. Specifically,the capping parts are utilized for blank ejection for preventingejection failure, in which the inks are ejected to the cap parts locatedin a state separated from the ink ejection openings. Further, the capparts are utilized for sucking the inks from the ink ejection openingsin a capped state by a pump (not illustrated) to recover ejection ofejection openings undergone ejection failure.

An ink receiving part 33 plays the role of receiving ink dropletspreliminarily ejected when the recording heads 211 to 214 pass throughover it just before recording operation. A blade or wiping member (notillustrated) is arranged at a position adjoining the cap parts, wherebythe ink ejection opening-forming faces of the recording heads 211 to 214can be cleaned.

As described above, it is preferable to add the recovery unit forrecording heads and preliminary units to the construction of therecording apparatus because the recording operation can be morestabilized. Specific examples of these units include capping units,cleaning units and pressurizing or sucking units for recording heads,preliminary heating units by electrothermal converters, other heatingelements than these converters or combinations thereof. It is alsoeffective for stably conducting recording to provide a preliminaryejection mode that ejection separate from recording is conducted.

In addition, a cartridge type recording head, in which an ink tank isprovided integrally with the recording head itself described in theabove-described embodiment may also be used. Further, a replaceable chiptype recording head, in which electrical connection to an apparatus bodyand the feed of an ink from the apparatus body become feasible byinstalling it in the apparatus body, may also be used.

FIG. 2 illustrates the construction of the recording heads 211 to 214.In the drawing, the recording scan directions of the recording heads 211to 214 are directions indicated by arrows. A plurality of nozzles, i.e.,ejection openings, arranged in a direction substantially perpendicularto the recording scan direction is provided in each of the recordingheads 211 to 214. Each recording head ejects ink droplets atpredetermined timing from the respective ejection opening while beingmoved and scanned in a recording scan direction in the drawing, wherebyan image is formed on recording medium with a recording resolutionaccording to the arrangement density of the nozzles. At this time, therecording head may conduct recording operation in any direction of therecording scan directions. The recording operation may be conducted inany direction of forward and return directions.

The above-described embodiment is a recording apparatus of the serialtype that a recording head is scanned to conduct recording. However, arecording apparatus of the full-line type that a recording head having alength corresponding to the width of a recording medium is used may alsobe used. As a recording head of the full-line type, there isconstruction that such recording heads of the serial type as disclosedin FIG. 2 are arranged in a zigzag state or in parallel to form acontinuous recording head so as to give the intended length.Alternatively, construction that one recording head integrally formed soas to have a continuous nozzle row is used may also be adopted.

The above-described recording apparatus of the serial type or line typeis an embodiment that a head of the construction of 4 ejection openingrows (or nozzle rows) independently or integrally formed and using 4color inks (Y, M, C and K) is installed. It is also preferable that thenumber of ejection opening rows is increased to about 5 to 12, and aboutat least one ink of the 4 color inks, inks of the same color areduplicatively charged in plural ejection opening rows (or nozzle rows).Examples thereof include construction of 8 ejection opening rows (ornozzle rows) and construction of 12 ejection opening rows (or nozzlerows) that 2 or 3 heads of the construction illustrated in FIG. 2 arecontinuously connected.

According to the ink jet recording apparatus of the present invention,the application of the ink is divided into plural times when an imagehaving a portion where the duty is 80% or more and the total amount ofan ink applied is 5.0 μl/cm² or less is formed in a basic matrix forforming the image. The amount of the ink applied at each of the timesdivided is controlled to 0.7 μl/cm² or less. The ink jet recordingapparatus of the present invention has a control mechanism forconducting such divided application of ink. The operation of the ink jetrecording head and the timing of conveyance operation of plain paper arecontrolled by this control mechanism to conduct such divided applicationof ink.

The number of times of division in the application of the ink may be setaccording to desired recording conditions. An example where theapplication of ink is divided into 2 times is illustrated in FIG. 3.This example is an example where the resolution of a basic matrix is1,200 dpi (width)×1,200 dpi (length), and an image having a portionwhere the duty is 100% is formed. In FIG. 3, the impact positions of theink applied at the first time and the impact positions of the inkapplied at the second time are illustrated as the first ink and thesecond ink, respectively. The first ink and second ink are respectivelyapplied in a fixed amount.

EXAMPLES

The present invention will hereinafter be described specifically by thefollowing Examples and Comparative Examples. Incidentally, alldesignations of “part” or “parts” and “%” in the following examples meanpart or parts by mass and % by mass unless expressly noted. An averageparticle size was measured by means of Nanotrac UPA 150EX (manufacturedby NIKKISO; as a 50% cumulative value).

First of all, preparation processes of pigment dispersions contained ininks used in Examples and Comparative Examples are described.

(Preparation Example 1) (Preparation of Pigment Dispersion) <Preparationof Self-Dispersion Pigment Dispersion A>

After carbon black (100 g) having a specific surface area of 220 m²/gand a DBP oil absorption of 105 mL/100 g and p-aminobenzoic acid (34.1g) were fully mixed with water (720 g), nitric acid (16.2 g) was addeddropwise to the resultant mixture, and the mixture was stirred at 70° C.After 10 minutes, a solution with sodium nitrite (10.7 g) dissolved inwater (50 g) was added, and stirring was conducted for additionally one(1) hour. The resultant slurry was filtered through filter paper (tradename: Toyo Filter Paper No. 2 ; product of Advantec Co.), and theresultant pigment particles were fully washed with water and dried in anoven controlled to 90° C. A self-dispersion black pigment with ap-aminobenzoic group introduced into the surface of carbon black wasobtained by the above-described process. After this pigment was adjustedwith ion-exchanged water so as to give a pigment concentration of 10%,the pH of the resultant dispersion was adjusted to 7.5 with aqueousammonia. The dispersion was filtered by using a prefilter and a filterhaving a pore size of 1 μm in combination to obtain a self-dispersionpigment dispersion A.

<Preparation of Self-Dispersion Pigment Dispersion B>

A self-dispersion pigment dispersion B was obtained in the same manneras in the preparation of the self-dispersion pigment dispersion A exceptthat C.I. Pigment Yellow 74 was used in place of carbon black.

<Preparation of Self-Dispersion Pigment Dispersion C>

A self-dispersion pigment dispersion C was obtained in the same manneras in the preparation of the self-dispersion pigment dispersion A exceptthat C.I. Pigment Red 122 was used in place of carbon black.

<Preparation of Self-Dispersion Pigment Dispersion D>

A self-dispersion pigment dispersion D was obtained in the same manneras in the preparation of the self-dispersion pigment dispersion A exceptthat C.I. Pigment Blue 15:3 was used in place of carbon black.

<Preparation of Ink 1>

After the following components (100 parts in total) were mixed for 2hours, the resultant mixture was adjusted to pH 8.5 with aqueous ammoniaand filtered through a filter having a pore size of 2.5 μm to obtain anink 1. The surface tension of the ink was 29 mN/m, and the averageparticle size of the self-dispersion pigment was 130 nm.

-   -   Self-dispersion pigment dispersion A: 40 parts    -   Glycerol (coefficient of hydrophilicity-hydrophobicity: 0.11):        16 parts    -   1,6-Hexanediol (coefficient of hydrophilicity-hydrophobicity:        0.76): 4 parts    -   Isopropyl alcohol: 1 part    -   Ethylene oxide adduct of acetylene glycol (trade name: Olfin        E1010, product of Nisshin Chemical Industry Co., Ltd., HLB        value: 10 or more): 1 part    -   Triethylene glycol monobutyl ether: 0.5 part    -   Water: balance.

<Preparation of Ink 2>

After the following components (100 parts in total) were mixed for 2hours, the resultant mixture was adjusted to pH 8.5 with aqueous ammoniaand filtered through a filter having a pore size of 2.5 μm to obtain anink 2. The surface tension of the ink was 29 mN/m, and the averageparticle size of the self-dispersion pigment was 130 nm.

-   -   Self-dispersion pigment dispersion A: 40 parts    -   Glycerol (coefficient of hydrophilicity-hydrophobicity: 0.11): 8        parts    -   Trimethylolpropane (coefficient of        hydrophilicity-hydrophobicity: 0.31): 8 parts    -   1,2-Hexanediol (coefficient of hydrophilicity-hydrophobicity:        0.97): 4 parts    -   Isopropyl alcohol: 1 part    -   Ethylene oxide adduct of acetylene glycol (trade name: Olfin        E1010, product of Nisshin Chemical Industry Co., Ltd., HLB        value: 10 or more): 1 part    -   Triethylene glycol monobutyl ether: 0.5 part    -   Water: balance.

<Preparation of Ink 3>

An ink 3 was obtained in the same manner as in the preparation of theink 1 except that the content of the ethylene oxide adduct of acetyleneglycol was changed from 1 part to 0.1 part. The surface tension of theink was 36 mN/m, and the average particle size of the self-dispersionpigment was 125 nm.

<Preparation of Ink 4>

An ink 4 was obtained in the same manner as in the preparation of theink 1 except that glycerol was changed to 1,2-pentanediol (coefficientof hydrophilicity-hydrophobicity: 0.93). The surface tension of the inkwas 28 mN/m, and the average particle size of the self-dispersionpigment was 135 nm.

<Preparation of Ink 5>

An ink 5 was obtained in the same manner as in the preparation of theink 1 except that 1,6-hexanediol was changed to triethylene glycol(coefficient of hydrophilicity-hydrophobicity: 0.07). The surfacetension of the ink was 30 mN/m, and the average particle size of theself-dispersion pigment was 130 nm.

<Preparation of Ink 6>

An ink 6 was obtained in the same manner as in the preparation of theink 1 except that the self-dispersion pigment dispersion B was used inplace of the self-dispersion pigment dispersion A. The surface tensionof the ink was 29 mN/m, and the average particle size of theself-dispersion pigment was 125 nm.

<Preparation of Ink 7>

An ink 7 was obtained in the same manner as in the preparation of theink 1 except that the self-dispersion pigment dispersion C was used inplace of the self-dispersion pigment dispersion A. The surface tensionof the ink was 29 mN/m, and the average particle size of theself-dispersion pigment was 85 nm.

<Preparation of Ink 8>

An ink 8 was obtained in the same manner as in the preparation of theink 1 except that the self-dispersion pigment dispersion D was used inplace of the self-dispersion pigment dispersion A. The surface tensionof the ink was 29 mN/m, and the average particle size of theself-dispersion pigment was 105 nm.

Examples 1 to 10 and Comparative Examples 1 to 9

The respective inks of Preparation Example 1 were used to provideexamples of image forming methods and recording apparatus in Examples 1to 10 and Comparative Examples 1 to 9 according to conditions shown inTable 2 and Table 3. In each example, the ink was charged in a black inkhead part of an ink jet recording apparatus. Table 2 shows exampleswhere the amount of the ink applied to an image was divided into equalamounts for respective times. Table 3 shows examples including caseswherein the amount of the ink applied to an image was changed for everytime. The “total amount applied” in Tables 2 and 3 is a total amount ofthe ink applied to the image upon the final formation of the image.

Office Planner paper (product of Canon Marketing Japan Inc.) that isplain paper for common use in PPC/BJ was used for evaluation of recordedimages. Ink jet recording apparatus used are the following 3 apparatus.

-   -   F930 (manufactured by Canon Inc.; recording head: 6 ejection        opening rows, including 512 nozzles in each row; amount of the        ink: 4.0 pl (fixed amount); maximum resolution: 2,400 dpi        (width)×1,200 dpi (length); hereinafter referred to as “Printer        A”)    -   W6200 (manufactured by Canon Inc.; recording head: 6 ejection        opening rows, including 1,280 nozzles in each row; amount of the        ink: 8.5 pl (fixed amount); maximum resolution: 1,200 dpi        (width)×1,200 dpi (length); hereinafter referred to as “Printer        B”)    -   F950 (manufactured by Canon Inc.; recording head: 6 ejection        opening rows, including 512 nozzles in each row; amount of the        ink: 2.0 pl (fixed amount); maximum resolution: 2,400 dpi        (width)×2,400 dpi (length); hereinafter referred to as “Printer        C”)

In this example of image formation, the resolution of a basic matrix wasset to 1,200 dpi×1,200 dpi for Printer A, 600 dpi×1,200 dpi for PrinterB and 2,400 dpi×1,200 dpi for Printer C.

TABLE 2 Amount applied Total amount Divided at each time applied InkPrinter times (μl/cm²) (μl/cm²) Ex. 1 2 A 2 0.5 1.0 Ex. 2 2 A 3 0.33 1.0Ex. 3 2 A 4 0.25 1.0 Ex. 4 2 A 8 0.125 1.0 Ex. 5 1 A 4 0.25 1.0 Ex. 6 2A 8 0.375 3.0 Ex. 7 2 A 8 0.4 3.2 Comp. 2 A 1 1.0 1.0 Ex. 1 Comp. 3 A 40.25 1.0 Ex. 2 Comp. 4 A 4 0.25 1.0 Ex. 3 Comp. 5 A 4 0.25 1.0 Ex. 4Comp. 2 A 8 0.65 5.1 Ex. 5 Comp. 2 B 4 0.25 1.0 Ex. 6 Comp. 2 B 4 0.52.0 Ex. 7

TABLE 3 Total Amount applied at each amount Divided time (μl/cm²)applied Ink Printer times 1st 2nd 3rd 4th (μl/cm²) Ex. 8 2 C 4 0.3 0.30.3 0.3 1.2 Ex. 9 2 C 4 0.2 0.1 0.1 0.7 1.2 Ex. 10 2 C 4 0.5 0.1 0.1 0.51.2 Comp. 2 C 4 0.75 0.2 0.15 0.1 1.2 Ex. 8 Comp. 2 C 4 0.1 0.15 0.20.75 1.2 Ex. 9

Recorded articles in Examples 1 to 10 and Comparative Examples 1 to 9were evaluated as to image density (O.D.), strike-through property,small character printing and resistance to clogging. The results areshown in Table 4. Evaluation as to images was made by using a black headand printing an image (3 cm×3 cm) of 100% duty and JIS first levelChinese characters of 5 point as small characters. With respect to theresistance to clogging, the apparatus was left to stand for 1 month atordinary temperature in a capped state after the investigation under therespective conditions to observe printing condition after recoveryoperation. Incidentally, evaluating methods and standards of therecording are as follows.

(Image Density)

O.D. of a solid print image was measured by a densitometer (MacbethRD915; manufactured by Macbeth Co.).

-   -   A: O.D. was 1.40 or more;    -   B: O.D. was 1.35 or more, but less than 1.40;    -   C: O.D. was 1.30 or more, but less than 1.35;    -   D: O.D. was less than 1.30.

(Strike-Through Property)

After a solid print image was printed, the degree of permeation into theback surface of paper was visually evaluated according to the followingevaluation standard.

-   -   A: Occurrence of strike through was not observed;    -   B: Occurrence of strike through was slightly observed;    -   C: Strike through was observed;    -   D: Strike through was conspicuously observed.

(Small Character Printing)

The sharpness of small characters (Chinese characters) printed wasvisually evaluated according to the following evaluation standard.

-   -   A: Disorder of outlines of characters is not observed even in        complex small characters;    -   B: Outlines of characters are slightly disordered in complex        small characters;    -   C: Outlines of characters are disordered in complex small        characters;    -   D: Disorder of characters may be observed even in simple small        characters in some cases.

(Resistance to Clogging)

Evaluation was made by printing a nozzle check pattern after recoveryoperation.

-   -   A: Normal printing can be conducted by one cleaning operation        (operation that an ink is sucked at ejection openings of a        recording head);    -   B: Normal printing cannot be conducted by one cleaning        operation, but normal printing can be conducted by three        cleaning operations;    -   C: Normal printing cannot be conducted by three cleaning        operations, but normal printing can be conducted by three strong        cleaning operations (operation that an ink is more strongly        sucked at ejection openings of a recording head than the        cleaning operation);    -   D: Neither stable ejection nor normal printing can be conducted        even when strong cleaning operation is conducted repeatedly.

TABLE 4 Strike- Small Resistance Image through character to densityproperty printing clogging Ex. 1 B B B A Ex. 2 A A A A Ex. 3 A A A A Ex.4 A A A A Ex. 5 A A B B Ex. 6 A A A A Ex. 7 A A B A Ex. 8 A A A A Ex. 9B B B A Ex. 10 A A A A Comp. Ex. 1 D D D A Comp. Ex. 2 C B A A Comp. Ex.3 C C A D Comp. Ex. 4 A A D A Comp. Ex. 5 C C D A Comp. Ex. 6 C C D AComp. Ex. 7 A D D A Comp. Ex. 8 A D D A Comp. Ex. 9 A C C A

When Examples 1 to 4 are compared with Comparative Example 1, it isunderstood that when the ink according to the present invention is usedand the application of the ink in the formation of an image is divided,image density, strike-through property and small character printing aremade very good. When Example 5 is compared with Comparative Example 2,it is understood that when the surface tension of the ink is controlledto 34 mN/m or less, image density is made good. When Example 5 iscompared with Comparative Examples 3 and 4, it is understood that whenthe water-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more and the water-solublecompound having a coefficient of hydrophilicity-hydrophobicity of 0.25or less are used in combination in the ink, image density,strike-through property, small character printing and resistance toclogging are made good at the same time. When Examples 1 to 7 arecompared with Comparative Example 5, it is understood that when therecording method according to the present invention is applied to animage that the final total amount of the ink applied is 5.0 μl/cm² orless, image density, strike-through property and small characterprinting are made good. When Examples 1 to 7 are compared withComparative Examples 6 and 7, it is understood that when the ink isejected in a fixed amount of from 0.5 pl or more to 6.0 pl or less,image density, strike-through property and small character printing aremade good. When Examples 8 to 10 are compared with Comparative Examples8 and 9, it is understood that when the amount of the ink applied ateach time in the formation of the image is 0.7 μl/cm² or less,strike-through property and small character printing are made good.

Examples 11 and 12, and Comparative Examples 10 to 12

The inks of the respective Preparation Examples and Printer A were usedto provide examples of color image- forming methods and recordingapparatus in Examples 11 and 12, and Comparative Examples 10 to 12according to combinations of 4 color inks and conditions shown in Table5.

Office Planner paper (product of Canon Marketing Japan Inc.) that isplain paper for common use in PPC/BJ was used for evaluation of recordedimages.

TABLE 5 Amount applied Total Ink charged in head at each amount K Y M CPrint- Divided time applied head head head head er times (μl/cm²)(μl/cm²) Ex. 11 1 6 7 8 C 4 0.25 1.0 Ex. 12 2 6 7 8 C 4 0.25 1.0 Comp. 36 7 8 C 4 0.25 1.0 Ex. 10 Comp. 4 6 7 8 C 4 0.25 1.0 Ex. 11 Comp. 5 6 78 C 4 0.25 1.0 Ex. 12

The recorded images of Examples 11 and 12, and Comparative Examples 10to 12 were evaluated as to bleeding and resistance to clogging. Theresults are shown in Table 6. With respect to the bleeding, solid printimages were printed with all the 6 combinations of the 4 inks chargedunder the same conditions in such a manner that the respective colorsadjoin each other, and the condition of the boundary portions thereofwas visually observed. With respect to the resistance to clogging, theapparatus was left to stand for 1 month at ordinary temperature in acapped state after the investigation under the respective conditions toobserve printing condition after recovery operation. Incidentally,evaluating methods and standards of the recording are as follows.

(Bleeding)

-   -   A: Bleeding is not observed;    -   B: Bleeding is slightly observed;    -   C: Bleeding is observed;    -   D: Bleeding is markedly observed.

(Resistance to Clogging)

Evaluation was made by printing a nozzle check pattern after recoveryoperation.

-   -   A: Normal printing can be conducted by one cleaning operation        (operation that an ink is sucked at ejection openings of a        recording head);    -   B: Normal printing cannot be conducted by one cleaning        operation, but normal printing can be conducted by three        cleaning operations;    -   C: Normal printing cannot be conducted by three cleaning        operations, but normal printing can be conducted by three strong        cleaning operations (operation that an ink is more strongly        sucked at ejection openings of a recording head than the        cleaning operation);    -   D: Neither stable ejection nor normal printing can be conducted        even when strong cleaning operation is conducted repeatedly.

TABLE 6 Resistance to Bleeding clogging Example 11 A B Example 12 A AComp. Example 10 C A Comp. Example 11 A D Comp. Example 12 D A

When Examples 11 and 12 are compared with Comparative Example 10, it isunderstood that when the surface tension of the ink is controlled to 34mN/m or less, bleeding is well inhibited. When Examples 11 and 12 arecompared with Comparative Examples 11 and 12, it is understood that whenthe water-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more and the water-solublecompound having a coefficient of hydrophilicity-hydrophobicity of 0.25or less are used in combination in the ink, bleeding and clogging arewell inhibited.

This application claims the benefit of Japanese Patent Applications No.2007-191038, filed Jul. 23, 2007 and No. 2008-036151, filed Feb. 18,2008, which are hereby incorporated by reference herein in theirentirety.

1. An ink jet image-forming method for forming an image by applying anink in a fixed amount of from 0.5 pl or more to 6.0 pl or less to plainpaper, wherein: the ink comprises a self-dispersion pigment, water, awater-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more as defined by thefollowing equation (A) and a water-soluble compound having a coefficientof hydrophilicity-hydrophobicity of 0.25 or less as defined by thefollowing equation (A), and has a surface tension of 34 mN/m or less,and the application of the ink is divided into plural times when animage having a portion where the ink is applied in a duty of 80% or moreand in an amount of 5.0 μl/cm² or less in total is formed in a basicmatrix for forming the image, and the amount of the ink applied at eachof the times divided is 0.7 μl/cm² or less:Coefficient of hydrophilicity-hydrophobicity=[(Water activity value of a20% aqueous solution)−(Molar fraction of water in the 20% aqueoussolution)]/[1-(Molar fraction of water in the 20% aqueous solution)].  Equation (A)
 2. The ink jet image-forming method according to claim 1,wherein the resolution of the basic matrix is from 600 dpi or more to4,800 dpi or less.
 3. The ink jet image-forming method according toclaim 1, wherein the number of times of the ink application divided isfrom 2 times or more to 8 times or less.
 4. The ink jet image-formingmethod according to claim 1, wherein the average particle size of theself-dispersion pigment is from 60 nm or more to 145 nm or less.
 5. Theink jet image-forming method according to claim 1, wherein theapplication of the ink is conducted by the action of thermal energy. 6.An ink jet color image-forming method for forming an image by applyingeach of at least 4 color inks of black, cyan, magenta and yellow in afixed amount of from 0.5 pl or more to 6.0 pl or less for each ink toplain paper, wherein: each of the 4 color inks comprises aself-dispersion pigment, water, a water-soluble compound having acoefficient of hydrophilicity-hydrophobicity of 0.37 or more as definedby the following equation (A) and a water-soluble compound having acoefficient of hydrophilicity-hydrophobicity of 0.25 or less as definedby the following equation (A), and has a surface tension of 34 mN/m orless, and the application of the inks is divided into plural times whenan image having a portion where the inks are applied in a duty of 80% ormore and in an amount of 5.0 μl/cm² or less in total is formed in abasic matrix for forming the image, and the amount of the inks appliedat each of the times divided is 0.7 μl/cm² or less:Coefficient of hydrophilicity-hydrophobicity=[(Water activity value of a20% aqueous solution)−(Molar fraction of water in the 20% aqueoussolution)]/[1-(Molar fraction of water in the 20% aqueous solution)].  Equation (A)
 7. The ink jet color image-forming method according toclaim 6, wherein the resolution of the basic matrix is from 600 dpi ormore to 4,800 dpi or less.
 8. The ink jet color image-forming methodaccording to claim 6, wherein the number of times of the ink applicationdivided is from 2 times or more to 8 times or less.
 9. The ink jet colorimage-forming method according to claim 6, wherein the average particlesize of the self-dispersion pigment is from 60 nm or more to 145 nm orless.
 10. The ink jet color image-forming method according to claim 6,wherein the application of the inks is conducted by the action ofthermal energy.
 11. An ink jet recording apparatus equipped with arecording head for forming an image by applying an ink in a fixed amountof from 0.5 pl or more to 6.0 pl or less to plain paper, wherein: theapparatus comprises a control mechanism, by which the application of theink is divided into plural times when an image having a portion wherethe ink is applied in a duty of 80% or more and in an amount of 5.0μl/cm² or less in total is formed in a basic matrix for forming theimage, and the amount of the ink applied at each of the times divided iscontrolled to 0.7 μl/cm² or less, and the ink comprises aself-dispersion pigment, water, a water-soluble compound having acoefficient of hydrophilicity-hydrophobicity of 0.37 or more as definedby the equation (A) and a water-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.25 or less as defined by the equation(A), and has a surface tension of 34 mN/m or less:Coefficient of hydrophilicity-hydrophobicity=[(Water activity value of a20% aqueous solution)—(Molar fraction of water in the 20% aqueoussolution)]/[1 -(Molar fraction of water in the 20% aqueous solution)].  Equation (A)
 12. An ink jet recording apparatus equipped with arecording head for forming an image by applying each of at least 4 colorinks of black, cyan, magenta and yellow in a fixed amount of from 0.5 plor more to 6.0 pl or less for each ink to plain paper, wherein: theapparatus comprises a control mechanism, by which the application of theinks is divided into plural times when an image having a portion wherethe inks are applied in a duty of 80% or more and in an amount of 5.0μl/cm² or less in total is formed in a basic matrix for forming theimage, and the amount of the inks applied at each of the times dividedis controlled to 0.7 μl/cm² or less, and each of the inks comprises aself-dispersion pigment, water, a water-soluble compound having acoefficient of hydrophilicity-hydrophobicity of 0.37 or more as definedby the equation (A) and a water-soluble compound having a coefficient ofhydrophilicity-hydrophobicity of 0.25 or less as defined by the equation(A), and has a surface tension of 34 mN/m or less:Coefficient of hydrophilicity-hydrophobicity=[(Water activity value of a20% aqueous solution)—(Molar fraction of water in the 20% aqueoussolution)]/[1-(Molar fraction of water in the 20% aqueous solution)].  Equation (A)
 13. The ink jet recording apparatus according to claim12, wherein the recording head is a recording head where the applicationof the inks are conducted by the action of thermal energy.
 14. The inkjet recording apparatus according to claim 11, wherein the recordinghead is a recording head where the application of the ink is conductedby the action of thermal energy.